DIRECTIONSIn a medium bowl, mix together the soy sauce, rice vinegar and brown sugar until the sugar is completely dissolved. Add the ginger, green onions, pepper flakes and vegetable oil. Place the cubes of tuna in the bowl, coat completely with the marinade. Cover and refrigerate for 1 to 2 hours.Remove tuna from marinade. Place marinade in a small saucepan and bring to a boil; simmer for 10 minutes, set aside.Thread tuna, pineapple and onion pieces on skewers. If using a grill, use high direct heat. Oil the grate. Place skewers on grill. Cover. If using an oven broiler, keep broiling pan 6 inches from the element. Turn after 2 to 4 minutes. Baste with reserved marinade. Cook for 2 to 4 minutes more, basting frequently until tuna is just barely cooked through.Serves 4.